1. Field of the Invention
The present invention relates to a tube structure of a multitubular heat exchanger, the heat exchange performance of which is enhanced and the flow resistance in the tube of which is reduced.
2. Description of the Related Art
Conventionally, in order to enhance the performance of a multitubular heat exchanger such as an EGR gas cooler or an exhaust heat recovery device for a co-generator in which fluid of low Prandtl Number such as water, air or exhaust gas is used as a medium, for example, in order to enhance the performance of a heat exchanger: in which a large number of tubes for cooling EGR gas are arranged in parallel (This heat exchanger will be referred to as an EGR cooler hereinafter.), as shown in FIGS. 16 to 18, protrusions protruding to the center of a tube are provided on the inner face of the tube at regular intervals in the axial direction. These protrusions will be referred to as beads in this specification, hereinafter.
Concerning the form of protruding the beads 2 from the inner surface of the tube 1, according to the method of press forming the beads 2, the following two cases are provided. One is a case in which the beads 2 are two-dimensionally protruded from the inner face of the tube on the circumference as shown in FIG. 16. The other is a case in which the beads 3 are spirally protruded from the inner face of the tube as shown in FIG. 17 or Unexamined Japanese Patent Publication No. 2000-345925. There is a small difference between the performance of these two cases.
The beads 2, 3 protruding from the inner face of the tube are bodies for facilitating the generation of a turbulent flow in the fluid flowing in the tube. Therefore, the heat transfer effect of the beads 2, 3 is high. However, when a flow rate of the exhaust gas is increased, the pressure loss in the tube is also increased.
Further, there is provided a tube structure in which the spiral fin 4 is arranged in the tube 1 having the beads 2 so that the heat radiating performance can be enhanced as shown in FIG. 18. This spiral fin 4 contributes to the enhancement of the heat radiating performance. However, an increase in the pressure loss in the tube is caused when this spiral fin 4 is arranged in the tube.
Therefore, it is desired to develop a tube structure capable of satisfying both the enhancement of the heat radiating performance and the reduction of the pressure loss in the tube so that the tube structure can meet the needs in the future.